Implantable stimulation devices are devices that generate and deliver electrical stimuli to body nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder sublaxation, etc. The present invention may find applicability in all such applications, although the description that follows will generally focus on the use of the invention within a spinal cord stimulation system, such as that disclosed in U.S. Pat. No. 6,516,227 (“the '227 patent”), issued Feb. 4, 2003 in the name of inventors Paul Meadows et al., which is incorporated herein by reference in its entirety.
Spinal cord stimulation is a well-accepted clinical method for reducing pain of certain populations of patients. A spinal cord stimulation (SCS) system typically includes an implantable pulse generator and at least one stimulation electrode lead that carries electrodes that are arranged in a desired pattern and spacing to create an electrode array. Individual wires within the electrode lead(s) connect with each electrode in the array. The electrode lead(s) is typically implanted along the dura of the spinal cord, with the electrode lead(s) exiting the spinal column, where it can generally be coupled to one or more electrode lead extensions. The electrode lead extension(s), in turn, are typically tunneled around the torso of the patient to a subcutaneous pocket where the implantable pulse generator is implanted. Alternatively, the electrode(s) lead may be directly coupled to the implantable pulse generator. For examples of other SCS systems and other stimulation systems, see U.S. Pat. Nos. 3,646,940 and 3,822,708, which are hereby incorporated by reference in their entireties.
Of course, implantable pulse generators are active devices requiring energy for operation. Oftentimes, it is desirable to recharge an implanted pulse generator via an external charger, so that a surgical procedure to replace a power depleted implantable pulse generator can be avoided. To wirelessly convey energy between the external charger and the implanted pulse generator, the recharger typically includes an alternating current (AC) charging coil that supplies energy to a similar charging coil located in or on the implantable pulse generator. The energy received by the charging coil located on the implantable pulse generator can then be used to directly power the electronic componentry contained within the pulse generator, or can be stored in a rechargeable battery within the pulse generator, which can then be used to power the electronic componentry on-demand.
To provide efficient power transmission through tissue from the external charger to the implanted pulse generator, it is paramount that the charging coil located in or on the implantable pulse generator be spatially arranged relative to the corresponding AC coil of the external charger in a suitable manner. That is, efficient power transmission through the patient's skin from the external charger to the implantable pulse generator via inductive coupling requires constant close alignment between the two devices. To ensure that such constant close alignment is achieved, the external charger typically includes an alignment indicator that provides a visual or audible signal that can be used by the patient to reposition or reorient the external charger, thereby maintaining or optimizing the rate at which the implantable pulse generator is charged.
One known approach is to use a charge strength indicator on the external charger to indicate the extent of the charge rate. For example, a bar charge indicator can be used, such that one bar indicates a relatively low charge rate, two bars indicate a greater charge rate, three bars indicate an even greater charge rate, and so forth. One downfall of using a bar charge connection indicator is that the patient must continually looks at the indicator to ensure an optimal charge rate.
Another approach is to use a misalignment indicator on the external charger that signals to the patient with an audible misalignment tone whenever the charge rate falls below the optimal level. However, this approach currently limits the possibility of charging more deeply implanted pulse generators at lower rates without inadvertently triggering the misalignment tone. Although the alignment zone of the external charger could be expanded to prevent such inadvertent triggering of the misalignment tone, the indicator may not generate the misalignment tone when the charge rate actually is less than optimal. Thus, the patient may charge the implantable pulse generator at a sub-optimal rate without ever being warned.
An external charger that combines both a bar charge indicator and a misalignment indicator would still require the patient to monitor the bar charge indicator during charging or endure an audible tone that inappropriately signals for deeper implantable pulse generators. There, thus, remains a need for an improved method and system for indicating alignment or misalignment between an external charger and an implantable pulse generator.